In the production process of stainless steel seamless pipe, it is necessary to pay attention to the production characteristics of the pipe. As long as these characteristics are fully utilized, the quality of the stainless steel pipe produced can be as good as possible.
Plastic characteristics of stainless steel pipe
The martensitic stainless steel is not difficult to perforate when it is a single austenite structure at high temperature, but the perforation performance is reduced when there is biphasic structure. pipe blank heating temperature is usually 1130-1160 °C, the furnace tail temperature is lower than 900 °C, the final rolling temperature is greater than the critical point transition temperature, low carbon is not less than 850 °C, high carbon is not higher than 925 °C. With the increase of carbon content in martensitic stainless steel, the strength and hardness of stainless steel increase, and the plasticity decreases. When cold drawing, the core plug should be used as far as possible to draw. This is because it is easy to burst after empty drawing.
The high temperature plasticity of austenitic stainless steels is related to the residual α phase. Plasticity generally weakens as the α phase increases. In addition to the α phase, there are other residual phases in the austenitic stainless steel, such as various forms of carbides, intermetallic compounds, and the like. The plasticity of these extra relative stainless steel pipes has an influence, and the degree of influence depends on their quantity and status. When heated, the excess phase appears as a network distribution on the grain boundary, which will significantly deteriorate the plasticity of the stainless steel pipe. Austenitic stainless steel pipes have low plasticity at high temperatures and large deformation resistance. Special attention should be paid to the selection of reasonable deformation parameters and temperature parameters when using oblique rolling piercing. After the austenite stainless steel pipe is treated by solution treatment, it shows a single-phase austenite structure. Its characteristic is that σb is high, but the σs is low, the ductility and toughness are relatively large, and the cold deformation performance is good, so it is desirable The amount of deformation of the pass. However, the work hardening during cold deformation is large, so the processing performance is poor.
Ferritic stainless steel is a single-phase structure and has no critical transition point (phase transition). Therefore, the long heating time of the pipe and the high temperature increase the grain size seriously, resulting in a significant decrease in the plasticity of the steel. The maximum temperature of pipe heating is 1000 to 1060°C. Steel has good plasticity (perforation) in this temperature range. The temperature before the cold-drawing hammerhead is 700-850°C. When heating, it is necessary to prevent carburizing, and after hammering, water quenching is performed. Since ferritic chromium stainless steel has room temperature brittleness and poor cold working properties, it is easy to crack and pull out when cold drawing. Therefore, it is best to draw under a certain temperature condition (see the drawing of the pipe), and the pulling speed should not be too high. When cold-rolled ferritic stainless steel pipe, the degree of deformation of the pipe during rolling is not more than 40% to 48%. In later passes (after surface preparation and heat treatment), the degree of deformation can be increased to 55% to 65% depending on the degree of grain refinement. In order to avoid cracking of the steel pipe during rolling, the temperature rise of the pipe during rolling is advantageous, so in most cases the stainless steel pipe is rolled without the emulsion cooling tool and metal.
Stainless steel pipe deformation resistance
Both austenitic and martensitic stainless steel pipes have high deformation resistance, a relatively large work hardening tendency, and slow recrystallization at high temperatures. Therefore, special attention must be paid to equipment and motor capabilities when cold-rolling such stainless steel pipes. Ferrite or hemiferous low-carbon stainless steel pipes have the same deformation resistance as ordinary carbon steel.
The wide spread of stainless steel pipes refers to the degree of transverse deformation, which is generally large. The width of martensitic stainless steel pipes is 1.3 times that of carbon steel, that of austenitic stainless steel is 1.35 to 1.5 times, and that of ferritic stainless steel is 1.55. -1.6 times. Therefore, it is necessary to pay attention to controlling the horizontal deformation degree of the stainless steel pipe during the oblique rolling and piercing, and to obtain a smaller ellipticity. The cold and hot-rolled stainless steel pipe has a larger ellipticity of the hole shape, and should take a smaller amount of deformation in order to prevent the occurrence of defects such as ears. In the case of no tension reducing, the wall thickness of the stainless steel pipe is relatively large and decreases. The hole machine ovality to take larger.
Ferritic stainless steel has brittleness at room temperature, cold processing is highly sensitive to stress, and the stress sensitivity during hot rolling is low, and water quenching can be performed after rolling. Martensitic stainless steels are more sensitive to cracks. Cooling after hot rolling can not be too fast. Usually, pile cold is used. Austenitic stainless steels have low sensitivity to cracks and can be water quenched after hot rolling.
The low-temperature thermal conductivity of stainless steel pipes is relatively poor, while the linear expansion coefficient is greater than that of carbon steel. In order to ensure the heating quality of the stainless steel pipe, low-temperature heating at a low temperature is used.
Because the stainless steel pipe material has good oxidation resistance, the scale formed during heating is relatively small. However, the iron oxide skin contains chromium and nickel oxides and is very dense, and it is difficult to carry out pickling during cold working. Usually HF+HNO3 is used for pickling or alkaline pickling.
Stainless steel ferritic, duplex stainless steel and austenitic stainless steel are susceptible to stick-rolling during rolling and reduce the surface quality of the stainless steel pipe. The use of cast iron rollers in the hot-rolled finishing mill can reduce sticking. In addition, the rational use of cooling water to cool the rollers is also critical.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)